There is increased interest in using perovskite materials as device components in semiconductor structures, instead of, or in addition to, more conventional materials such as silicon or gallium arsenide. Perovskites are transition metal oxides capable of forming a cubic lattice structure and have a general chemical formula of ABO3 where A and B are cations, and the atoms of A-type cations have a larger diameter than the atoms of the B-type cations. The unit cell of the cubic structure for perovskites have oxygen atoms located at the faces of the cube, a first cation type (e.g., A-type atoms) located at the corners of the cube and a second cation type located (e.g., B-type atoms) in the center of the cube. The chemical structure of perovskites are well known to those skilled in the art and therefore need not be described in further detail.
Certain types of perovskites have been used in nonvolatile memory cells where the perovskite material serves as a polarizable ferroelectric material situated between two conducting plates. Information may be stored in the memory cell by passing an electric current through the conducting plates to generate an electrical field to change the internal polarization of the ferroelectric perovskite material.
There are problems, however, in using ferroelectric perovskites to form such semiconductor structures. For instance, the use of ordered ferroelectric perovskite material in memory cells is desirable because such material has a larger net electric dipole than amorphous ferroelectric perovskite materials. However, conducting plates made of conventional metals, such as platinum, are not conducive to the fabrication of ferroelectric perovskites that have an ordered crystal structure. Moreover it is difficult to grow ordered ferroelectric perovskite crystals on a template comprised of such metals because the metals have a polycrystalline or substantially amorphous structure. As a result, the ferroelectric perovskites formed thereon do not have a sufficiently large net electric dipole for efficient storage of information. Moreover, memory cells having such ferroelectric perovskite materials deposited on a metal plate have a high fatigue factor, meaning that they rapidly lose their ability to be polarized after a few cycles of exposures to alternating electrical fields.
It has also proven difficult to prepare ordered conductive perovskite materials that could be used as a conductive plate instead of conventional metals. In particular, previous preparations of conductive perovskite materials do not provide a smooth ordered layer to serve as a template for the deposition of an ordered perovskite ferroelectric. As a result, similar to that discussed above for metal conductive plates, the perovskite ferroelectric material does not have a sufficiently large net polarization to efficiently store information.
Strontium ruthenate (SrRuO3) perovskites while being good conductors, degrade during subsequent steps in the preparation of the memory cell, such as steps involving exposure to high temperatures (e.g., greater than about 500° C.). In particular, because the Ru atoms are volatile, the SrRuO3 perovskite becomes depleted of Ru, thereby losing its conductive properties. In addition, it has proven difficult to grow a uniform ordered layer of SrRuO3 perovskites.
Another example of a conductive perovskite is lanthanum-doped strontium titanate (SrTiO3), where the lanthanum atoms partially replace the A-type cations in a random fashion, to give an intermixed Lax-1SrxTiO3 perovskite. The conductivity of such perovskites in thin film form, however, is rapidly lost when the Lax-1SrxTiO3 perovskite is exposed to processing steps involving high temperatures and high oxygen partial pressure (e.g., about one Torr).
Accordingly, an objective of the invention is to produce conductive perovskite material that is resistant to oxidation and therefore suitable for use in semiconductor structures without encountering the above-mentioned difficulties.